1. Field of the Invention
Exemplary aspects of the present invention generally relate to an electrophotographic image forming apparatus, such as a copier, a facsimile machine, a printer, or a multi-functional system including a combination thereof.
2. Description of the Related Art
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile capabilities, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of an image bearing member (which may, for example, be a photoconductive drum); an optical writer projects a light beam onto the charged surface of the image bearing member to form an electrostatic latent image on the image bearing member according to the image data; a developing device supplies toner to the electrostatic latent image formed on the image bearing member to render the electrostatic latent image visible as a toner image; the toner image is directly transferred from the image bearing member onto a recording medium or is indirectly transferred from the image bearing member onto a recording medium via an intermediate transfer member; a cleaning device then cleans the surface of the image carrier after the toner image is transferred from the image carrier onto the recording medium; finally, a fixing device applies heat and pressure to the recording medium bearing the unfixed toner image to fix the unfixed toner image on the recording medium, thus forming the image on the recording medium.
In recent years, a variety of recording media sheets such as paper having a luxurious, leather-like texture and Japanese paper known as “Washi” have come on the market. Such recording media sheets have a coarse surface through an embossing process to produce that luxurious impression. However, toner does not transfer well to such embossed surfaces, in particular the recessed portions of the surface. This improper transfer of the toner appears as dropouts or white spots in the resulting output image.
Various attempts have been made to prevent improper transfer of the toner under such circumstances. For example, according to JP-2008-185890-A, a recording medium is heated immediately before a toner image is transferred thereon, and the recording medium is charged with a polarity opposite that of the toner. In this configuration, a transfer electric field is enhanced so that the toner is transferred to the recessed portions of the recording medium. However, the desired transferability is still not achieved if the recessed portions are relatively deep.
In another approach, in order to prevent dropouts and obtain desired imaging quality, an alternating current (AC) voltage is superimposed on a direct current (DC) voltage to form a transfer bias. For example, in JP-2006-267486-A, a superimposed bias, in which an AC voltage is superimposed on a DC voltage, is used as the transfer bias, and the surface of the recording medium is charged with a polarity opposite that of the toner in accordance with the roughness of the surface prior to transfer.
The superimposed transfer bias may have several permutations. For example, In JP-2008-058585-A, as the transfer bias, the AC voltage is superimposed on the DC voltage such that a peak-to-peak voltage of the AC voltage is equal to or less than twice the DC voltage. In JP-H09-146381-A, a surface of an intermediate transfer member employs a fluorocarbon resin, and as the transfer bias, the AC voltage is superimposed on the DC voltage such that the peak-to-peak voltage of the AC voltage is 2.05 times the DC voltage or greater. In JP-H04-086878-A, as the transfer bias, the AC voltage is superimposed on the DC voltage such that the frequency of the AC voltage is 4 kHz or less and the number of cycles in a transfer nip is 20 or more.
Although the above-described approaches are advantageous and generally effective for the intended purpose, the level of the superimposed AC voltage is relatively low so that the toner does not transfer well onto the recessed portions of the recording media. In order to overcome this difficulty, as the transfer bias, the AC voltage is superimposed on the DC voltage, and the peak-to-peak value of the AC voltage can be 4 times the absolute value of the DC voltage. In this configuration, the transferability can be improved, but depending on the surface condition of recording media sheets, image defects including horizontal streaks still appear in an output image.
In view of the above, there is thus an unsolved need for an image forming apparatus capable of maintaining good transferability regardless of surface conditions of recording media sheets.